Xenacoelomorpha are a group of small, enigmatic marine invertebrates that have captivated scientists due to their unusual characteristics and challenging position within the animal kingdom. Their simple body plans and unique evolutionary history make them a subject of ongoing scientific investigation. Understanding these organisms offers insights into the early diversification of animal life and helps reconstruct the tree of life.
What Are Xenacoelomorpha?
Xenacoelomorpha are small, flat, worm-like creatures, typically a few millimeters long. They inhabit marine environments, often found in sediment or between sand grains, and some species can even be found near hydrothermal vents at depths of nearly 4 kilometers. All known species are bilaterally symmetrical, meaning they have distinct front, back, left, and right sides, a characteristic of most complex animals.
Despite their bilateral symmetry, Xenacoelomorpha possess a simple body plan compared to many other bilaterally symmetrical animals. They are triploblastic, meaning their bodies develop from three embryonic germ layers, but they lack a true body cavity (coelom), making them acoelomate. Their digestive system is a simple internal cavity or a solid mass of tissue, and they lack a true anus, expelling food waste through the mouth.
The nervous system in Xenacoelomorpha is simple and diffuse, located just under the epidermis. Unlike many other bilaterians, they do not possess a centralized brain or ganglia, though some acoelomorphs show neural aggregations in the anterior region. They also lack a specialized excretory system, although genes related to such systems are present.
Their Unique Place in the Tree of Life
The phylogenetic placement of Xenacoelomorpha has been a subject of debate among scientists. Historically, based on their superficial resemblance, they were often grouped with flatworms (Platyhelminthes). However, molecular studies have revealed that they are distantly related to flatworms and hold a more ancient position in the animal tree.
Early molecular analyses suggested that Xenacoelomorpha might be basal deuterostomes, a group that includes more complex animals like vertebrates, sea urchins, and starfish. This idea proposed that their simple body plan might be a result of secondary simplification, meaning they evolved from more complex ancestors and then lost certain features. This challenged the traditional view that their simplicity reflected an ancestral state for all bilaterians.
Recent phylogenomic analyses often place Xenacoelomorpha as a sister group to all other bilaterians, or as the basalmost bilaterian clade. This position implies that their simple morphology could represent characteristics of the earliest bilaterally symmetrical animals, offering a window into how complex body plans might have first evolved. The ongoing debate highlights the challenges in reconstructing deep evolutionary relationships, especially when dealing with organisms that have undergone significant evolutionary changes or simplifications.
Life and Habits
Xenacoelomorpha navigate their marine environments through ciliary gliding. Their entire epidermis is covered in cilia, tiny hair-like structures that propel them across sediment or through water. A layer of mucus, secreted by ventral glands, aids in this gliding locomotion.
Their feeding strategies vary, but involve engulfing food particles. Smaller species feed by extending their syncytial gut through their mouth to engulf diatoms, small invertebrates, or detritus. Larger Xenoturbella species feed on mollusks or decomposed fragments of dead animals. Some species also exhibit symbiotic relationships, such as the Pacific waminoa, which harbors symbiotic dinoflagellates.
Reproduction in Xenacoelomorpha is sexual, and all species are hermaphroditic, meaning each individual possesses both male and female reproductive organs. They undergo direct development, bypassing a larval stage, with offspring resembling miniature adults upon hatching. While Xenoturbella species engage in external fertilization, acoelomorphs exhibit internal fertilization.
Diversity Within Xenacoelomorpha
The phylum Xenacoelomorpha comprises two sister groups: Acoelomorpha and Xenoturbellida. Acoelomorpha is further divided into two subgroups: Acoela and Nemertodermatida. The phylum includes approximately 456 described species.
Acoela is the most diverse subgroup, with around 400 species. These animals are characterized by their simple gut, which lacks a distinct lumen, meaning ingested food goes directly into the internal tissues. Nemertodermatida, with about 18 species, are distinguishable by a statocyst containing two statoliths, which are small mineral concretions involved in balance. Xenoturbellida comprises a smaller number of species, such as Xenoturbella bocki and Xenoturbella westbladi. They are larger than acoelomorphs, with some species reaching up to 4 centimeters.